This invention relates to filters and, in particular, to systems and methods for use in implementing tunable ceramic filters in wireless communication systems.
Conventional constant percent bandpass filters are typically limited to around 2% and, at most, for some frequencies around 1%. However, these filters are inapplicable for certain applications where the percent of bandwidth is required to be less than 1%, such as less than 0.1% and more preferably less than around 0.08% as shown in FIG. 7. Although it may be possible to tune a conventional percent bandpass filter to have a narrow 3 db bandwidth, the insertion loss becomes higher than an acceptable level such as 3 db. For example, if a conventional constant percent bandpass filter is tuned to have less than 0.1% 3 db bandwidth, the insertion loss may be as high as 6 db or more. Accordingly, a new filter design is required for certain applications such as cellular base station testing applications and other suitable applications.
With reference to FIG. 9, an example of a conventional air wave guide tunable filter 300 is shown. The air wave guide tunable filter 300 may include a plurality of waveguide cavities 302 each having a capacitive tuning plunger 308 interconnected via a series of gears 301 and a knob 302 for turning the gears 301. The plunger 308 is a double helical metal plunger providing an RF short in the cavity 302 which makes the waveguide cavity appear smaller as the plunger is turned down into the cavity. Thus, it appears to the RF signal as if the cavity ceiling was made shorter. The cavities are connected at the outside via an input connector 304 and an output connector 305. Each of the cavities may also include a fine tuning adjustment screw 306. The airwave guide tunable filters 300 are capable of having small percentage 3 db bandwidth filters, but are not easily scalable to low frequencies. For example, a three-cavity air wave guide filter for a one gigahertz signal may be required to have, for example, a plurality of nine inch cavities such as three nine inch cavities connected in series. Accordingly, these filters are not desirable in that they are large, bulky, and expensive to manufacture. The larger nine inch plungers are problematic in that they must be machined to very high tolerances to provide the correct RF short, and thus the larger plunger sizes are problematic to machine at these close tolerances. Referring to FIGS. 10-11, another type of conventional tunable filter is termed an "air variable capacitor tunable filter" or air variable capacitance tuner 200. The air variable capacitance tuner 200 includes a single resonator 204 in a cavity 202 with a capacitive plate 201 that may be adjusted to have a variable distance from the resonator 204. A capacitor plate may fit into a slot 203 in the resonator 204 and be adjusted to either be closer to or further away from the resonator 204. The variable capacitance tuners 200 have poor insertion loss when tuned to a narrow band 3 db bandwidth, and are therefore undesirable for some applications.
Waveguide cavity filters may be of a fixed configuration or of a tunable configuration. FIG. 8 illustrates a conventional dielectric loaded wave guide cavity that may be tuned to a higher frequency by moving the metal plate 330 lower in the cavity and closer to the ceramic puck 331. The problem with tuning the cavity of FIG. 8 is that as the metal disk is lowered closer and closer to the ceramic puck, to produce a higher resonant frequency, the Q of the cavity decreases substantially. Although the Q does not effect the 3 db bandwidth which is still tunable, the reduction in the Q has a substantial impact on the insertion loss of the wave guide filter.
An alternative tunable filter is shown in FIG. 12, where the ceramic puck 331 may be tuned by lowering a dielectric disk 332 closer to the puck. The lowering of the dielectric disk lowers the frequency of the wave guide cavity. The problem with tuning the cavity of FIG. 12 is that as the dielectric disk 332 is lowered closer and closer to the ceramic puck 331 in order to produce a lower resonant frequency, the Q of the cavity decreases substantially. The reduction in the Q has a substantial impact on the insertion loss of the wave guide filter. Conventionally, the dielectric disks are used for fine tuning and not for severely altering the center frequency of the bandpass filter over a wide range.
A problem arises with conventional tunable waveguide cavity filters in that none of these filters provides a suitable configuration which allows severely altering the center frequency of a bandpass filter over a wide range while still maintaining an acceptable insertion loss.